Conventionally, encapsulation of various substances has been performed in many industrial and technical fields. In the industry of printing, coating material and ink, a large number of techniques for encapsulating a pigment, a coloring matter or the like have been practiced. Also, in the medical or agricultural field, many attempts to encapsulate a drug have been made for the purpose of increasing the efficacy, decreasing the toxicity, imparting the stability, sustaining the effect, or the like. As for the encapsulation method, a phase separation method (coacervation method), a submerged drying method (inter-facial precipitation method), a spray drying method, a pan coating method, a submerged curing coating method, an interfacial polymerization method, an interfacial inorganic reaction method, an in-situ polymerization method and the like are known. However, these methods have a problem, for example, the core substance is limited, the thickness of the shell layer (encapsulating layer of the core substance) is hard to freely design, the encapsulation of one core substance is difficult, the functional group on the capsule surface is hard to freely design, a particle having a uniform surface state cannot be easily produced, encapsulation on the nano-order is not easy, application to a relatively unstable compound is difficult, a solvent used at the production of a preparation is readily mixed into the product, or the property of the obtained capsule is not satisfied. Also, the resulting encapsulated product itself has a problem depending on the usage.
In an inkjet recording method of jetting out an ink droplet from a fine nozzle head and recording a letter or a figure on the surface of a recording medium such as paper, an aqueous pigment ink obtained by dispersing a pigment in water has recently come to be used because of its excellent water resistance or light fastness. As for such an aqueous pigment ink, those obtained by dispersing a pigment in an aqueous dispersion medium with use of a dispersant such as surfactant or polymer dispersant are generally used in many cases. However, when a dispersant is used for the dispersion of pigment particles, the ink composition has many points to be adjusted for ensuring preferred properties as an ink, for example, there is a problem that when high printing density, fixing property or scratch resistance is intended to obtain, the viscosity tends to be high.
Furthermore, in such an aqueous pigment ink, the dispersant is merely adsorbing to the pigment particle surface and in the inkjet recording method where a strong shear force is applied to the pigment particle, the dispersant adsorbing to the pigment particle surface is sometimes desorbed, as a result, the dispersibility of the pigment ink may be decreased to deteriorate the ejection stability (property that the ink is stably jetted out in a constant direction from a recording head). Also, in a pigment ink using a pigment dispersed by using a dispersant such as surfactant or polymer dispersant, desorption and absorption of the dispersant readily occurs and when the ink is stored for a long time, the dispersion is liable to become unstable.
On the other hand, for the purpose of enhancing the fixing property of the pigment contained in a pigment-type inkjet ink on a recording medium, a technique of using an encapsulated pigment where the colorant particle is encapsulated with a polymer, is known.
Those prepared by encapsulating a pigment particle (see, for example, Patent Documents 1, 2 and 3) or those prepared by graft-polymerizing a polymer to the surface of a pigment particle (see, for example, Patent Documents 4 to 7) have been proposed. Also, a method of encapsulating a hydrophobic powder by using an amphipatic graft polymer has been proposed (see, for example, Patent Document 8), but this method has a problem that when a previously polymerized polymer is used for the encapsulation, the particle size after encapsulation becomes excessively large.
Other than these proposals, there has been proposed an ink using a pigment on which a resin capable of forming a film at room temperature is encapsulated by a phase inversion emulsification method (see, for example, Patent Documents 9 to 17), or an ink using a pigment on which an anionic group-containing organic polymer compound is encapsulated by an acid dipping method (see, for example, Patent Documents 18 to 27).
In addition, an ink using a polymer emulsion in which a fine polymer particle and a coloring material are impregnated by a phase inversion emulsification method has been proposed (see, for example, Patent Documents 28 to 33). However, even in a colorant obtained by a phase inversion emulsification method or an acid dipping method, the polymer adsorbed to the pigment particle may desorb and dissolve in the ink depending on the kind of an organic solvent such as penetrant, and the dispersion stability or ejection stability of ink, the image quality and the like are sometimes unsatisfied.    Patent Document 1: JP-B-7-94634 (the term “JP-B” as used herein means an “examined Japanese patent publication”)    Patent Document 2: JP-A-8-59715 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)    Patent Document 3: JP-A-2003-306661    Patent Document 4: JP-A-5-339516    Patent Document 5: JP-A-8-302227    Patent Document 6: JP-A-8-302228    Patent Document 7: JP-A-8-81647    Patent Document 8: JP-A-5-320276    Patent Document 9: JP-A-8-218015    Patent Document 10: JP-A-8-295837    Patent Document 11: JP-A-9-3376    Patent Document 12: JP-A-8-183920    Patent Document 13: JP-A-10-46075    Patent Document 14: JP-A-10-292143    Patent Document 15: JP-A-11-80633    Patent Document 16: JP-A-11-349870    Patent Document 17: JP-A-2000-7961    Patent Document 18: JP-A-9-31360    Patent Document 19: JP-A-9-217019    Patent Document 20: JP-A-9-316353    Patent Document 21: JP-A-9-104834    Patent Document 22: JP-A-9-151342    Patent Document 23: JP-A-10-140065    Patent Document 24: JP-A-11-152424    Patent Document 25: JP-A-11-166145    Patent Document 26: JP-A-11-199783    Patent Document 27: JP-A-11-209672    Patent Document 28: JP-A-9-286939    Patent Document 29: JP-A-2000-44852    Patent Document 30: JP-A-2000-53897    Patent Document 31: JP-A-2000-53898    Patent Document 32: JP-A-2000-53899    Patent Document 33: JP-A-2000-53900